Currently available technologies for curing or molding polymers typically use a heated mold that supplies heat to polymeric bulk by conduction. This is inefficient, non-uniform, difficult to control precisely and leads to defective and unsatisfactory parts. For instance, when curing a polymer or composite part in a mold, the heat is conducted from the external surfaces, leading to long process times and unacceptable residual stresses within the final product.
Ferromagnetic particles (susceptors) are known to rapidly convert incident energy into heat at well dispersed sites at polymeric interfaces. Magnetic susceptors are used to convert energy derived from radio frequency induction heaters to heat energy at a point of application. For example, U.S. Pat. No. 5,529,708 (Palmgren et al.) discloses a method for preparing a hot melt adhesive by incorporating magnetic susceptor particles in a hot melt adhesive matrix and subjecting the susceptor particles to a magnetic induction field thereby melting the adhesive matrix.
Conventional induction heating uses micron-sized particles, typically from 1-50 microns. Known particles may contain iron(II,III) oxide, zinc doped iron oxide, nickel, and cobalt. The heating capability of these particles is conventionally expected to reduce significantly as the particle size decreases.
Current research on induction susceptor particles focuses on tailoring particles so that their Curie temperatures approach the target curing temperature of the samples. Curie temperature is the temperature where a material's permanent magnetism changes to induced magnetism—above this temperature these susceptor particles would lose their heating capability. Modifying the Curie temperature is tricky, unpredictable and involves complicated chemistry.